T. rex was a huge meat-eating dinosaur that lived during the late Cretaceous period, about 85 million to 65 million years ago. T. rex lived in a humid, semi-tropical environment, in open forests with nearby rivers and in coastal forested swamps. The seasons were mild.

ANATOMY
Tyrannosaurus rex was a fierce predator that walked on two powerful legs. This meat-eater had a huge head with large, pointed, replaceable teeth and well-developed jaw muscles. It had tiny arms, each with two fingers. Each bird-like foot had three large toes, all equipped with claws (plus a little dewclaw on a tiny, vestigial fourth toe). T. rex had a slim, stiff, pointed tail that provided balance and allowed quick turns while running. T. rex's neck was short and muscular. Its body was solidly built but its bones were hollow.

SIZE
Tyrannosaurus rex was up to 40 feet (12.4 m) long, about 15 to 20 feet (4.6 to 6 m) tall. The arms were only about 3 feet (1 m) long. Tyrannosaurus rex was roughly 5 to 7 tons in weight.

The enormous skull was about 5 feet (1.5 m) long. The eye sockets in the skull are 4 inches (10.2 cm) across; the eyeballs would have been about 3 inches (7.6 cm) in diameter.

T. rex left footprints 1.55 feet (46 cm) long (although its feet were much longer, about 3.3 feet (1 m) long; T. rex, like other dinosaurs, walked on its toes). It had a stride length of up to 12 to 15 feet (3.7-4.6 m). T. rex may have run at up to 15 mph (24 kph).

TEETH AND JAWST. rex's jaws were up to 4 feet (1.2 m) long and had 50 to 60 thick, conical, bone-crunching teeth that ranged in size from very small to over 9 inches (23 cm) long. Adult had a variety of sizes of teeth in their jaws at one time, as teeth were broken and new (smaller) ones grew in to replace them. One T. rex was found with some teeth up to 13 inch (33 cm) long. T. rex could eat up to 500 pounds (230 kg) of meat and bones in one bite!

Tyrannosaurus rex had a wrap-around overbite; when T. rex closed its mouth, the upper parts of the lower jaw's teeth fit inside the upper teeth.

SKIN
Fossilized specimens of T. rex's rough, scaly skin have been found. It was bumpy, like an alligator's skin, and has been described as a "lightly pebbled skin."

HABITAT AND DISTRIBUTION
Tyrannosaurus rex probably lived in forests, where its prey (plant-eating dinosaurs) could find plenty of food. T. rex fossils have been found in western North America and Mongolia.

SENSESSight: T. rex had large visual lobes in its brain that processed visual information. T. rex also had depth perception (since both eyes faced forwards on the front of its skull, and not placed on the sides), but it was not the only dinosaur that had depth perception. In general, predators (hunters) ofter have depth perception to help them hunt their prey. Animals that are hunted (like the plant-eating dinosaurs) usually have eyes located on the sides of their head (having no depth perception); this lets them see predators approaching from both sides.

Smell: T. rex's brain had a very large area in the brain for processing odors.

TAIL
Tyrannosaurus had a stiff, pointed tail (like other Tetanurans [meaning "stiff tail"]). The tail was used as a counterbalance for its enormous head, for agility and for making quick turns.

The rear part of the tail was stiffened by interlocking vertebral zygopophyses (interlocking bony structures projecting forwards and backwards from the neural arches, interlocking one vertebra into another).

OTHER HUGE MEAT-EATING DINOSAURS
Although not the biggest meat-eating dinosaur ever discovered, Tyrannosaurus rex was certainly one of the largest terrestrial carnivores of all time. The recently discovered Giganotosaurus carolinii and Carcharodontosaurus may have been even more enormous.

Too big to fail?Don't worry, we're too big to fail.
Cope's Rule (named for the American paleontologist Edward Drinker Cope) states that organisms within a population evolve to become more massive over time. Although this increases each individual's fitness, it leaves the species more susceptible to extinction.

The Lilliput Effect (named by Adam Urbanek, 1993) notes the appearance of small body size in surviving animals after an extinction event. The name Lilliput is from Jonathan Swift's novel Gulliver's Travels; in the novel, the Lilliputians were very tiny people.